THE LIGHT-FIELD OF MICROBENTHIC COMMUNITIES - RADIANCE DISTRIBUTION AND MICROSCALE OPTICS OF SANDY COASTAL SEDIMENTS

The light field in coastal sediments was investigated at a spatial res olution of 0.2-0.5 mm by spectral measurements (450-850 nm) of field r adiance and scalar irradiance using fiber-optic microprobes. Depth pro files of field radiance were measured with radiance microprobes at rep resentative angles relative to vertically incident collimated light in rinsed quartz sand and in a coastal sandy sediment colonized by micro algae. Upwelling and downwelling components of irradiance and scalar i rradiance were calculated from the radiance distributions. Calculated total scalar irradiance agreed well with the scalar irradiance measure d directly by a fiber-optic scalar irradiance microprobe. Close to the sediment surface, the light field was highly anisotropic, dominated b y incident collimated light, and the scalar irradiance reached a maxim um of 200% of incident scalar irradiance. Below the sediment surface, the light field became diffuse with a forward-biased angular light dis tribution. A few millimeters into the sediment surface, attenuation co efficients of field radiance, irradiance, and scalar irradiance became identical and independent of depth, indicating that the light field a pproached an asymptotic radiance distribution. Comparison of light fie lds in wet and dry quartz sand showed that the lower refractive index of air than of water caused a more forward-biased scattering in wet sa nd. Light penetration was therefore deeper and surface irradiance refl ectance was lower in wet sand than in dry sand. The higher reflectance of dry sand resulted in a higher surface maximum of scalar irradiance . Asymptotic values of average cosines and attenuation coefficients we re used to calculate the absorption coefficient in quartz sand and in a coastal sediment with diatoms. Absorption coefficients ranged from 2 .5 mm-1 at 450 nm to 1.5 mm-1 at 850 nm in the coastal sediment and fr om 0.8 to 0.4 mm-1 in wet quartz sand. Both attenuation spectra and ab sorption spectra of the coastal sediment with diatoms exhibited a Chl a absorption maximum at 675 nm. The light field around microalgae in s ediments differs strongly from the incident light field with respect t o intensity and spectral composition.